1. Field of the Invention
The present invention relates to a laser lap welding process of welding together plated steel sheets for forming automotive bodies or the like.
2. Description of the Related Art
Galvanized steel sheets, i.e., surface-treated steel sheets, have been used in the automobile industry. However, galvanized steel sheets must be galvanized in a large coating weight because zinc coating steel sheets is relatively rapidly corroded.
Hot-dip galvanealed steel sheets are prevalently used. A hot-dip galvanealed steel sheet is manufactured by plating a steel sheet with molten zinc by hot dipping to form a zinc-coated steel sheet, subjecting the zinc-coated steel sheet to an alloying process to suppress the activity of the zinc coating by forming an alloy layer on the surface of the steel sheet through the mutual diffusion of Fe and Zn. The coating weight of the zinc coating of such a hot-dip galvanealed steel sheet, as compared with that of the electrolytic zinc coating steel sheet, can be economically increased. Therefore, the enhancement of the corrosion resistance of the hot-dip galvanealed steel sheet can be easily achieved by increasing the coating weight.
When welding together overlapped zinc plated steel sheets including the hot-dip galvanealed steel sheets by laser lap welding, the coating layers coating the joined surfaces of the overlapped plated steel sheets produce a large amount of zinc vapor and the zinc vapor is sealed in a molten metal. Consequently, the sealed zinc vapor causes the formation of many weld defects that spoil beads, including blow holes in beads, depressions in the surfaces of beads and pits penetrating beads.
Many laser lap welding techniques that form a gas discharge passage between the joining surfaces of the overlapped plated steel sheets have been tried. A laser lap welding process disclosed in, for example, JP-A No. Hei 11-226765 forms irregularities by blasting in the joining surfaces of overlapped plated steel sheets to enable a vapor of a component of the coating layers coating the joining surfaces vaporized by laser lap welding to escape outside through the irregularities.
However, the vapor is unable to escape completely before melted parts of the plated steel sheets solidify when the height of the gap between the joining surfaces, i.e., the distance between the joining surfaces, is excessively small and the aforesaid weld defects are formed in welded parts. On the other hand, the molten metal flows into the gas discharge passage and parts of the plated steel sheet irradiated with a laser beam melt away and the same plated steel sheet is ruptured when the height of the gap is excessively great.
Therefore, when welding together overlapped plated steel sheets by laser lap welding, the height of the gap between the joining surfaces of the plated steel sheets (the distance between the joining surfaces of the plated steel sheets) that define the gas discharge passage must be severely adjusted by using a special bracket, which increases steps of work and reduces production efficiency.
The present invention has been made in view of the aforesaid problems in the related art and it is therefore an object of the present invention to provide a laser lap welding process of welding together overlapped plated steel sheets, capable of stably discharging outside a vapor of a component of the coating layers formed by plating and coating the joining surfaces of the overlapped plated steel sheets, of welding together the overlapped plated steel sheets in a stable weld quality and of improving production efficiency.
According to a first aspect of the present invention, a laser lap welding process of welding together overlapped first and second plated steel sheets comprises the steps of: forming slits intermittently in the first plated steel sheet to be irradiated with a laser beam along a weld line; and irradiating parts of the first plated steel sheet extending on the weld line and not provided with the slits with a laser beam to weld together the first and second plated steel sheets.
Preferably, the laser lap welding process in the first aspect of the present invention further comprises the step of removing parts of the coating layer of the second plated steel sheet respectively corresponding to the slits intermittently formed in the first plated steel sheet. Preferably, the same laser lap welding process further comprises the step of stopping the slits of the first plated steel sheet by filling the slits with a molten metal and irradiating the molten metal with a laser beam after welding together the first and second plated steel sheets. Preferably, the same laser lap welding process forms the slits intermittently by irradiating parts of the first plated steel sheet extending on the weld line with a laser beam.
The laser lap welding process according to the first aspect of the present invention forms the slits intermittently in the first plated steel sheet and irradiates the parts of the first plated steel sheet extending on the weld line and not provided with the slits with a laser beam to weld together the first and second plated steel sheets. Thus, a vapor produced in the joining surfaces of the plated steel sheets is able to escape outside through the slits intermittently formed in the first plated steel sheet. Consequently, the vapor does not remain in beads formed by laser lap welding, the formation of weld defects including blow holes and pits in the beads can be avoided and the plated steel sheets can be welded together in a stable weld quality. Since any special gas discharge passage does not need to be formed between the joining surfaces of the plated steel sheets, work for laser lap welding is simplified, weld quality is not affected by the shapes of the joining surfaces of the plated steel sheets because the joining surfaces of the plated steel sheets are kept in close contact with each other, the plated steel sheets can be welded together in a stable weld quality and production efficiency can be improved.
According to a second aspect of the present invention, a laser lap welding process of welding together overlapped first and second plated steel sheets comprises the steps of: inserting inserts between the joining surfaces of the first and second plated steel sheets to form a gas discharge passage between parts of the first and second plated steel sheets around a weld line; and irradiating parts of the first and second plated steel sheets extending on the weld line with a laser beam to weld together the first and second plated steel sheets.
In the second aspect of the present invention, it is preferable that the inserts are adhesive members attached to the parts of the first and second plated steel sheets near the weld line. Preferably, the laser lap welding process in the second aspect of the present invention further comprises the step of pressing the parts to which the adhesive members are attached by pressure rollers so that the gas discharge passage is formed in a predetermined height between the joining surfaces of the first and second plated steel sheets. Preferably, the adhesive members are placed on parts of the first and second plated steel sheets extending on the opposite sides of the weld line. Preferably, the adhesive members are separated from a release paper sheet unwound from a roll of release paper sheet and are attached to the joining surface of either of the first and second plated steel sheet.
The laser lap welding process according to the second aspect of the present invention inserts the inserts between the joining surfaces of the first and second plated steel sheets to form the gas discharge passage between the parts of the first and second plated steel sheets around the weld line and irradiates the parts of the first and second plated steel sheets extending on the weld line with a laser beam to weld together the first and second plated steel sheets. Thus, a vapor produced by the coating layers in the joining surfaces of the plated steel sheets is able to escape outside through the gas discharge passage. Consequently, the vapor does not remain in beads formed by laser lap welding, the formation of weld defects including blow holes and pits in the beads can be avoided and the plated steel sheets can be welded together in a stable weld quality. Since the gas discharge passage is formed between the joining surfaces of the plated steel sheets by the inserts, the plated steel sheets do not need to be processed by a special process to form the gas discharge passage and hence the laser lap welding process is simplified. Since the height of a part of the gas discharge passage corresponding to the weld line is dependent on the height of the inserts, the gas discharge passage can be held in an appropriate height during laser lap welding, the height of the gas discharge passage can be easily managed, the plated steel sheets can be welded together in a stable weld quality and production efficiency can be improved.
According to a third aspect of the present invention, a laser lap welding process of welding together overlapped first and second plated steel sheets comprises the steps of: partly deforming at least either of the first and second plated steel sheets so that a space is formed between the joining surfaces of the first and second plated steel sheets when the first and second plated steel sheets are joined, to form a gas discharge passage between parts of the first and second plated steel sheets near a weld line; and irradiating parts of the first and second plated steel sheets extending on the weld line with a laser beam to weld together the first and second plated steel sheets.
In the third aspect of the present invention, it is preferable to form the gas discharge passage by spacing apart the joining surfaces of the first and second plated steel sheets by protrusions formed in either of the first and second plated steel sheets. Preferably, the first and second plated steel sheets are spaced apart from each other by the protrusions so that the respective joining surfaces of the first and second plated steel sheets are inclined to each other and the parts of the first and second plated steel sheets extending on the weld line are spaced a predetermined distance apart from each other. Preferably, the gas discharge passage is formed by partly spacing the respective joining surfaces of the first and second plated steel sheets by a step formed in either of the first and second plated steel sheets. Preferably, the gas discharge passage is formed by partly spacing the respective joining surfaces of the first and second plated steel sheets by a flange having a curved bend formed in either of the first and second plated steel sheets.
The laser lap welding process according to the third aspect of the present invention partly deforms at least either of the first and second plated steel sheets so that a space is formed between the joining surfaces of the first and second plated steel sheets when the first and second plated steel sheets are joined, to form the gas discharge passage between the parts of the first and second plated steel sheets near the weld line and irradiates the parts of the first and second plated steel sheets extending on the weld line with a laser beam to weld together the first and second plated steel sheets. Thus, a vapor produced by the coating layers in the joining surfaces of the plated steel sheets is able to escape outside through the gas discharge passage. Consequently, the vapor does not remain in beads formed by laser lap welding, the formation of weld defects including blow holes and pits in the beads can be avoided and the plated steel sheets can be welded together in a stable weld quality. Since the gas discharge passage is formed between the joining surfaces of the plated steel sheets by the projections or the steps, the laser lap welding process does not need any special bracket, and hence the laser lap welding process is simplified. Since the height of a part of the gas discharge passage corresponding to the weld line is dependent on the height of the projections or the steps, the gas discharge passage can be held in an appropriate height during laser lap welding, the height of the gas discharge passage can be easily managed, the plated steel sheets can be welded together in a stable weld quality and production efficiency can be improved.